The present invention relates to a method for controlling a NOx removal system in a boiler intended to reduce NOx.
In recent years, there has been a desire for further NOx reduction also in boilers. One of the measures therefor is a method that with a NOx removal system provided in the boiler, ammonia as a reductant is supplied to discharge gas by the NOx removal system, thereby reducing the NOx. In this NOx removal system, ammonia is generated by heating urea water with ammonia generating means having an electric heater, and the amount of ammonia generation is controlled by adjusting the amount of urea water supplied to the ammonia generating means. The amount of ammonia supplied to discharge gas is controlled according to the combustion level of the boiler. For example, in a boiler in which combustion level is controlled in multiple stages of high combustion, low combustion and standby, the ammonia supply in the NOx removal system is also controlled in multiple stages in correspondence to the combustion levels.
In this connection, since piping of a specified length is provided to connect the ammonia generating means and jet nozzles for supplying ammonia to the discharge gas to each other, the ammonia supply cannot be changed over to a specified supply level unless the ammonia remaining in the piping is completely jetted out. This means that even if a changeover signal for combustion level and a changeover signal for ammonia supply are issued simultaneously, there will be a time lag until a changeover to a proper ammonia supply corresponding to the combustion level is actually effectuated. Therefore, in the case of a changeover of combustion level from high to low combustion level, it may occur that ammonia is supplied successively during the time lag so that part of the ammonia flows outside as it is, unfortunately.
In some cases, the NOx removal system is provided with a NOx removal catalyst for accelerating the reduction reaction of NOx by ammonia. In this NOx removal system, first the reductant is adsorbed by the NOx removal catalyst, and then NOx is decomposed into nitrogen and water promptly by a reduction reaction of NOx by the adsorbed reductant. Consequently, even with low-temperature discharge gas, NOx in the discharge gas can be reduced.
In this connection, in the NOx removal system provided with the NOx removal catalyst, there have been some cases where, at an increase of combustion level, for example, at a changeover of combustion level from low to high combustion level, unreacted part of ammonia flows outside as it is. In order to prevent such outflow of ammonia, it could be conceived to provide a large volume of the NOx removal catalyst, but this would incur upsizing of the NOx removal catalyst, inhibiting an efficient use of the NOx removal catalyst.
A first object of the present invention is to provide a method for controlling a NOx removal system in a boiler capable of preventing outflow of unreacted reductant from the boiler due to a delay from a changeover of combustion level of the boiler to a changeover of reductant supply.
A second object of the invention is to provide a method for controlling a NOx removal system in a boiler capable of preventing outflow of unreacted reductant from the boiler upon a changeover of combustion level of the boiler in the case where the NOx removal system is provided with a NOx removal catalyst.
In order to achieve the above second object, we inventors energetically made studies and experiments and so on with respect to the NOx removal system, having acquired the following finding. That is, the finding is that in order to prevent any leak of the reductant in increasing the combustion level, there is a need of adjusting the amount of reductant adsorption according to changes of the upper limit of reductant adsorption amount by the NOx removal catalyst (hereinafter, referred to as xe2x80x9csaturation adsorption amountxe2x80x9d). In more detail of this finding, since a reduction reaction of NOx in the NOx removal catalyst is caused by the reductant adsorbed by the NOx removal catalyst, a specified amount of reductant needs to have been adsorbed to achieve enough reduction of NOx. However, the reductant can only be adsorbed to the NOx removal catalyst up to the saturation adsorption amount. Also, the saturation adsorption amount decreases more and more with increasing temperature of the NOx removal catalyst, and increases more and more with decreasing temperature of the NOx removal catalyst. Therefore, when the temperature of discharge gas flowing through the NOx removal catalyst is increased due to an increase in combustion level, the NOx removal catalyst is heated by this discharge gas, causing the saturation adsorption amount to decrease. The result would be that the reductant is not adsorbed to the NOx removal catalyst or that adsorbed reductant is released out of the NOx removal catalyst, allowing unreacted reductant to be flowed out of the boiler. Accordingly, in order to prevent any leak of the reductant in increasing the combustion level, the reductant adsorption amount has to be decreased according to a reduction of the saturation adsorption amount. From this point of view, we inventors created a control method for solving the foregoing second object based on this finding.
The present invention having been achieved with a view to solving the foregoing issues, in a first aspect of the invention, there is provided a method for controlling a NOx removal system in which reductant supply is adjusted according to combustion level in a boiler in which the combustion level is controlled in multiple stages, the method comprising: for a decrease of the combustion level, keeping the reductant supply lower than a level corresponding to a changeover-target combustion level for a period of a set time from a changeover of the combustion level.
In a second aspect of the invention, there is provided a method for controlling a NOx removal system in which reductant supply is adjusted according to combustion level in a boiler in which the combustion level is controlled in multiple stages, the method comprising: for a decrease of the combustion level, changing over the reductant supply earlier than a changeover of the combustion level.
In a third aspect of the invention, there is provided a method for controlling a NOx removal system in which reductant supply is adjusted according to combustion level in a boiler in which the combustion level is controlled in multiple stages, where the reductant is reacted in a NOx removal catalyst, the method comprising: for an increase of the combustion level, keeping the reductant supply lower than a level corresponding to a changeover-target combustion level for a period of a set time from a changeover of the combustion level.
Further, in a fourth aspect of the invention, there is provided a method for controlling a NOx removal system in which reductant supply is adjusted according to combustion level in a boiler in which the combustion level is controlled in multiple stages, where the reductant is reacted in a NOx removal catalyst, the method comprising: for an increase of the combustion level, making and keeping the reductant supply at a lower level for a period of a set time lasting until a changeover of the combustion level.
Next, embodiments of the present invention are described. The invention is suitably embodied in a NOx removal system of a boiler. This boiler is, for example, a multi-tubular boiler, and the boiler body has a constitution in which, as an example, a combustion chamber is formed inside annular heat transfer tube arrays while an annular gas passage is formed outside the annular heat transfer tube arrays, where a flue is connected to the gas passage.
The NOx removal system is so constructed that the reductant is jetted out from jet nozzles provided at an outlet of the gas passage into combustion gas to thereby reduce the NOx in the combustion gas. In the case where the reductant is ammonia generated by heating and decomposing urea water, an ammonia generating means having a heating means such as electric heater is connected to the jet nozzles and the urea water supply to the ammonia generating means is controlled, by which the amount of ammonia jetted out from the jet nozzles is controlled. Other than urea water, compounds that are decomposed by heating to generate ammonia, such as cyanuric acid, melamine and biuret are also usable.
The boiler is so constructed that combustion level is controlled in multiple stages, e.g. three stages of high combustion, low combustion and standby, or four stages of high combustion, middle combustion, low combustion and standby. Therefore, in the NOx removal system, the reductant supply is controlled in multiple stages according to the combustion level of the boiler. That is, for three-stage control of combustion level, the reductant supply is controlled also in three stages, and for four-stage control of combustion level, the reductant supply is controlled also in four stages.
In the NOx removal system, the reductant supply for each level is set according to the NOx generation level corresponding to each combustion level of the boiler. For a changeover of combustion level, a changeover of the reductant supply is controlled as follows.
First, a first control method and a second control method are described. These control methods are those for solving the foregoing first issue. Firstly, in the first control method, for a decrease of combustion level, the reductant supply is kept smaller than a level corresponding to the changeover-target combustion level for a set time period starting at the changeover of combustion level. Then, after an elapse of the set time period, the reductant supply is changed over to the level corresponding to the changeover-target combustion level. By so doing, during the time lag from the issue of a reductant-supply-amount changeover signal to the actual changeover of the amount of the reductant jetted out from the jet nozzles, any excessive jet-out of reductant can be prevented so that the reductant can be prevented from flowing outside as it is unreacted. Otherwise, the supply of reductant may be halted for the set time period, where the reductant supply corresponding to the changeover-target combustion level is set to zero, depending on the circumstances of embodiment.
In the case where the first control method is executed, for an increase of combustion level, output of a combustion-level changeover signal and output of a reductant-supply-amount changeover signal are effected simultaneously. In this case, although the reductant supply becomes slightly short relative to the amount of NOx generation during the aforementioned time lag, yet this only lasts for a very short time without any problem.
Next, the second control method is described. In this second control method, for a decrease of combustion level, the reductant supply is changed over earlier than the combustion level is changed over. More specifically, a timing at which the amount of reductant jetted out from the jet nozzles is actually changed over is predicted by taking into consideration the time lag, and in order that the combustion level is changed over at this timing, output of the combustion-level changeover signal and output of the reductant-supply-amount changeover signal are effected with a specified time lag. By so doing, at a changeover of combustion level, any excessive jet-out of reductant can be prevented so that the reductant can be prevented from flowing outside as it is unreacted.
With regard to the specified time lag, since a changeover of combustion level in the boiler is performed normally based on steam pressure, the time at which the steam pressure will reach a set value for the changeover of combustion level can be predicted by detecting the variation gradient of steam pressure. Accordingly, a reductant-supply-amount changeover signal is outputted somewhat earlier than the predicted time. Otherwise, depending on the circumstances of embodiment, the output of the combustion-level changeover signal may be delayed by a specified time by outputting the reductant-supply-amount changeover signal when the steam pressure reaches the set value, and by outputting the combustion-level changeover signal in a specified time elapse after the steam pressure has reached the set value.
The second control method, although intended for use at a decrease of combustion level, may also be applied when the boiler is stopped from combustion and brought into a standby state. In this case, for a halt of the combustion of the boiler, a reductant-supply halt signal is outputted a specified time earlier than a combustion halt signal.
In the case where the second control method is executed, for an increase of combustion level, output of a combustion-level changeover signal and output of a reductant-supply-amount changeover signal are effected simultaneously. In this case, although the reductant supply becomes slightly short relative to the amount of NOx generation during the aforementioned time lag, yet this only lasts for a very short time without any problem.
Furthermore, the first control method and the second control method, usable each singly as they are, may also be used together in combination. The first control method and the second control method may be executed in this order for a changeover of combustion level. That is, for a decrease of combustion level, the reductant supply may be made smaller than a level corresponding to the changeover-target combustion level earlier than this changeover of combustion level, and maintained for a set time period from the changeover of combustion level.
As described above, according to the first control method and the second control method, for a decrease of combustion level, any excessive jet-out of reductant can be prevented so that the reductant can securely be prevented from flowing outside as it is unreacted.
Next, a third control method and a fourth control method of the invention are described. These control methods are those for solving the foregoing second issue and for cases in which the NOx removal system is provided with a NOx removal catalyst. The NOx removal catalyst has a function of accelerating the reduction reaction of NOx by a reductant, and is placed within the flue. Accordingly, when combustion gas mixed the reductant at the outlet of the gas passage has flowed as discharge gas along up to the NOx removal catalyst, the NOx reduction reaction by the NOx removal catalyst is accelerated by the NOx removal catalyst so that NOx is decomposed into nitrogen and water promptly, with the result of reduced NOx.
With further regard to the NOx removal catalyst, in order that the NOx reduction reaction is accelerated by the NOx removal catalyst, a specified amount of reductant needs to have been adsorbed in the NOx removal catalyst. In this connection, there is an upper limit for the amount of reductant adsorbed into the NOx removal catalyst, where this upper limit of adsorption (hereinafter, referred to as xe2x80x9csaturation adsorption amountxe2x80x9d) decreases with increasing temperature of the NOx removal catalyst and, conversely, increases with decreasing temperature of the NOx removal catalyst. Therefore, as the discharge gas passing through the NOx removal catalyst increases in temperature due to an increase in the combustion level of the boiler, the NOx removal catalyst is heated by the discharge gas, causing the saturation adsorption amount to decrease. However, the NOx removal catalyst is accelerated in its reduction reaction in proportion to increase of its temperature, thus capable of enough reduction of NOx even if the saturation adsorption amount has decreased.
Now the third control method is described. In the third control method, for an increase of combustion level, the reductant supply is kept smaller than a level corresponding to the changeover-target combustion level for a set time period starting at the changeover of combustion level. Then, after the changeover of combustion level, the reductant supply becomes slightly short relative to the amount of NOx generation, and this shortage is compensated by the reductant that has been adsorbed to the NOx removal catalyst, causing the reductant adsorption amount to decrease. After an elapse of the set time period, the reductant supply is changed over to the level corresponding to the changeover-target combustion level. The set time period is a value determined according to the time necessary for the NOx removal catalyst to decrease to the level corresponding to the decrease in the saturation adsorption amount after the changeover-target combustion level.
Thus, the adsorption amount can also be decreased according to the decrease in the saturation adsorption amount caused by the increase in the combustion level. Accordingly, when the combustion level is increased, the reductant can be prevented from flowing out of the boiler due to the reasons that the reductant is not adsorbed to the NOx removal catalyst or that the reductant that has been adsorbed to the NOx removal catalyst is released out.
Next, the fourth control method is described. In this fourth control method, for an increase of combustion level, the reductant supply is kept smaller for a set time period lasting to the changeover of combustion level. Then, when or after the combustion level is changed over, the reductant supply is changed over to the level corresponding to the changeover-target combustion level.
By so doing, before a changeover of combustion level, the reductant adsorption amount in the NOx removal catalyst can be decreased in advance. Therefore, even if the saturation adsorption amount has decreased due to an increase in combustion level, the reductant is adsorbed by the NOx removal catalyst so that the reductant that has been adsorbed to the NOx removal catalyst is no longer released out. Thus, the reductant can be prevented from flowing out of the boiler.
The third control method and the fourth control method, usable each singly as they are, may also be used together in combination. The third control method and the fourth control method may be executed in this order for a changeover of combustion level. That is, for an increase of combustion level, the reductant supply may be made smaller than a level corresponding to the changeover-target combustion level earlier than this changeover of combustion level, and maintained for a set time period from the changeover of combustion level.
Furthermore, in the NOx removal system provided with the NOx removal catalyst, for an increase of combustion level of the boiler, either one of the third control method or the fourth control method or both of them in combination are used for control, whereas for a decrease of the combustion level, either one of the first control method or the second control method or both of them in combination may be used for control.
In the above description, the reductant supply is decreased at a changeover of the combustion level. This decrease of supply amount includes a case where the reductant supply is set to zero.